Increasing throughput demands on leading edge scanners are requiring greatly improved light source availability. This can translate directly to minimizing downtime and maximizing productive time, as defined in the SEMI E10 standard. Focused efforts to achieve these goals are ongoing and have already yielded significant improvements on production light sources. One positive contributor to improving productive time can be minimization of the light source stoppage for halogen gas replenishment. Applicants' assignee's laser systems employ one or more halogen gas filled chambers as a gain medium. As the light source operates, the halogen gas is depleted and contaminants accumulate, so the gas must be periodically replenished.
This can be done by a partial replenishment while the light source continues to operate, called an inject, that is subject to constraints to ensure the light properties remain within pre-determined selected specifications for certain laser output parameters. Alternatively, it may be done with a full replenishment, called a refill, where all of the chamber gas is replaced while the laser is not firing to return the gas content in the laser to an originally selected pre-mix concentration such as 0.1% F2, 1.9% Ar and 98% neon. Refills are to be minimized because of the large disruption they introduce to both the light source and scanner operation. Continued pressures from the end users of the light sources, e.g., semiconductor manufacturers for increasingly narrow bandwidth and increasing pulse to pulse stability for bandwidth and a number of other beam quality parameters are influenced by many factors in the operation of such excimer laser photolithography DUV light sources. These include current gas composition and changes in gas composition over time, and also including issues of operating efficiency and economy, such as need to operate the scanner even while gas composition is being modified by a gas composition control system and down time due to periodic gas refill requirements. Certain weaknesses in one or more of the systems discussed in the above referenced issued patents and pending applications have thereby been exposed.
Applicants' assignee, Cymer, Inc. has proposed and adopted a number of laser gas control methods and apparatus both for single chamber and multi-chamber line narrowed laser systems, such as those that are utilized on semiconductor manufacturing photolithography as very narrow bandwidth, high pulse repetition rate, high power, extremely stable, pulse to pulse, DUV light sources, e.g., for use in scanners. For example, U.S. application Ser. No. 10/953,100, referenced above, describes how the amount of halogen to use in a gas replacement can be computed, and how it can be determined when the gas replacement occurs.
Along with improvements to gas management, major efforts in light source fault reduction, module lifetime extension and optimization of module replacement, can provide significantly increased combined light source\scanner availability. As throughput demands increase on leading edge scanners, a greater focus on minimizing downtime and maximizing productive time becomes essential. In the past, cutting edge light sources have focused primarily on delivering the high performance requirements demanded by the lithographic process. However light source manufacturers have an increasing responsibility to ensure that the light source delivers improved availability as the product matures. The SEMI E10 standard defines downtime to include preventative maintenance and replacement of consumables, such as light source chambers and optics. The SEMI E10 standard, named, Specification for Definition and Measurement of equipment Reliability, Availability, and Maintainability includes total time lost (downtime) due to module replacement and non-productive manufacturing standby time that includes halogen gas refills. Applicants' assignee Cymer has committed considerable effort to ensuring a positive trend to the light source availability is maintained. To date, interrupting the scanner operation to allow for full halogen gas replenishment of the light source has been an unavoidable necessity. However according to aspects of an embodiment of the disclosed subject matter, applicants propose better gas control algorithms, whereby fewer full halogen gas replenishments (replacements), which require the laser to stop discharging, may be needed, leading to appreciable gains in productive time.
Applicants' assignee Cymer's XLA and 7000 series lasers employ one or more halogen gas filled chambers as the gain medium. As the light source operates, the halogen gas is depleted and contaminants accumulate, so the gas must be periodically replenished.
The halogen gas may consist of either Argon (Ar) or Krypton (Kr) depending on the desired laser wavelength, along with, e.g., Neon (Ne) and also Fluorine (F2). As the light source discharges energy across its electrodes to generate Deep Ultra-Violet (DUV) light, some of the fluorine atoms may be temporarily disassociated and temporarily form dimers of ArF or KrF. They may then recombine with other compounds (e.g. metals) inside the light source chamber and possibly form solid particles that accumulate as debris within the chamber. This debris can have two negative effects: (1) reduction of the amount of fluorine available for use as a dielectric between the electrodes and (2) acting as a contaminant decreasing the light source efficiency. Other contaminants may also be present in the chamber gas including carbon compounds, atmospheric gases, and combinations of these molecules with fluorine. These compounds can manifest over time causing a decrease in the laser efficiency seen, e.g., as an increase in discharge voltage required to create a constant pulse energy. The discharge voltage has an upper limit and so action must be taken remove contaminants and replenish the lost fluorine, typically in the form of a full gas replenishment (refill).
The need for refills, as discussed previously, requires the light source to stop discharging light. When this happens, the lithographic process must be halted in a controlled manner to prevent reworking of the in-process wafers. This control is achieved by coordinating refills with the scanner. However, the need for a refill can depend on several complex and often unpredictable variables (light source firing pattern, light source energy, age of light source modules, etc.). Therefore, coordination of refills with the scanner is done by a regular schedule, which ensures that the light source operation will never suffer unanticipated interruption due to the light source reaching some operational limit. This schedule often yields very conservative upper limits on the time between refills. That is, if some users of the light source operate at low pulse usages, the actual time between a required refill could be much greater than the simple schedule permits. Applicants assignee has developed technology that more accurately predicts the need for a refill, to reduce this conservatism, and deliver longer gas lives on average.
Applicants propose certain improvements to certain aspects of the above referenced single chamber and multi-chamber laser gas control systems.